Recently, due to increasing data traffic, photonic networks, which are capable of high-capacity long-haul transmission have been introduced. Optical amplifiers are used, for example, to compensate for optical loss in optical transmitters/receivers and transmission loss in long-haul optical fiber cables. Examples of optical amplifiers are optical fiber amplifiers and semiconductor optical amplifiers (SOAs). SOAs are advantageous in that SOAs may amplify optical signals of different wavelengths that are used in photonic networks, by using different materials for the SOAs. Thus, SOAs are increasingly being used in photonic networks.
Low power consumption in addition to high optical output is desired for SOAs that are used in photonic networks. SOAs obtain optical gain by using stimulated emission that occurs in the active layer when current is injected into the active layer. The optical output of SOAs is dependent on the amount of injected current. Thus, obtaining high optical output with low injected current is desirable in reducing power consumption while achieving a high optical output. That is, the energy conversion efficiency in converting electric energy, which is injected into SOAs via current to optical energy, is preferably increased.
However, the energy conversion efficiency of SOAs is significantly lower than the energy conversion efficiency of optical fiber amplifiers, which are in practical use in photonic networks.
In conventional techniques, increasing the energy conversion efficiency of SOAs sufficiently while achieving a high optical output is difficult. Although reducing the polarization dependence of optical gain in SOAs that are used in photonic network is desirable, reducing the polarization dependence of optical gain in SOAs is considerably difficult using conventional techniques.
Japanese Patent Laid-open No. 03-284892, Japanese Patent Laid-open No. 04-217382, Japanese Patent Laid-open No. 07-135372, and Japanese Patent Laid-open No. 05-067845 are examples of related art documents.